1. Field of the Invention
The present invention relates to a synthetic fiber and, more particularly, to a blend of synthetic fibers for providing both structural and crack-controlling reinforcement to building materials.
2. Description of the Background of the Invention
It is well known that the addition of a reinforcement component to building materials such as cementitious materials, brick, asphalt, and the like improves the structural integrity of the material and reduces the likelihood of cracking. When incorporated into cementitious materials such as concrete, for example, the reinforcement component is added to reduce the effect of two main structural deficiencies: 1) low tensile strength; and 2) low strain at fracture. The tensile strength of concrete is relatively low because concrete, when formed, normally contains numerous micro-cracks. It is the rapid propagation of these micro-cracks under applied stress that is responsible for the low tensile strength of the material. Because of the widespread use and applicability of concrete, considerable research has been undertaken to lessen the effects of its deficient structural properties.
Typical reinforcement materials that are added to cementitious materials include, for example, various gauges of wire mesh or reinforcement fibers. A variety of reinforcement fiber additives are known in the art that provide strength characteristics to building materials. Typical reinforcement fibers include asbestos fibers, glass fibers, steel fibers, mineral fibers, and cellulose fibers. Some reinforcement fibers are better suited for particular applications than others. For example, asbestos fibers are known to provide effective reinforcement but, due to environmental and health concerns, are not extensively used. In addition, glass fibers and steel fibers are relatively expensive, and have a tendency to decompose in cementitious materials. Steel fibers typically decompose at the surface of the fiber reinforced material, whereas glass fibers continuously undergo decomposition as a result of the alkaline nature of cement. Also, due to the physical and chemical characteristics of steel fibers, there is some difficulty in uniformly distributing the steel fiber throughout the mixture. Furthermore, there are certain physical and operational deficiencies inherent with steel fiber that reduce its effectiveness. Such deficiencies include, for example, rebound in air-placed concrete applications, and relatively high equipment costs due to equipment wear from contact with the steel fibers.
It is known that concrete has a tendency to shrink after it has been cast due to the evaporation of excess mixing water. Plastic shrinkage causes the formation of shrinkage cracks shortly after the casting of the concrete, that weakens the matrix thereof. Unlike other fibrous materials, synthetic fibers are known to reduce such cracking caused by early plastic shrinkage. For example, a fibrillated fiber formed from a polyolefin film has been successfully used to prevent or reduce cracking. The fibers are stretched multiple times and then cut along lines at least partially transverse to the direction of orientation. The fibers are thereby fibrillated. When mixed within cementitious materials, in such a manner that they provide deformations to improve anchoring and bonding within the concrete matrix, the cut fibers are dispersed through the mixture, open to form webs nets, and thereby improve the strength and binding characteristics of the cementitious matrix.
Some advances have been made in the area of fiber reinforcement to provide increased toughness and durability, and reduce cracking in the matrix of building materials, such as concrete. However, the prior art reinforced fibers have a number of disadvantages that weaken or, otherwise, limit their effectiveness. Accordingly, there is a need for an improved reinforcement fiber that imparts improved structural properties to the building materials to which they are added. In particular, the need exists for a synthetic reinforcement fiber that when added to, for example, cementitious materials, provides a building material that exhibits reduced permeability, increased fatigue strength, improved toughness, and reduced plastic shrinkage.
The present invention provides a synthetic fiber blend that includes a first fiber component formed of a homopolymer polypropylene fiber, and a second fiber component. The second fiber component is a copolymer, formed of a polypropylene and a high density polyethylene. The polypropylene portion of the second fiber component is preferably present in a major amount, most preferably about 70 to 80 percent by weight, and the high density polyethylene portion is preferably present in a minor amount relative to the polypropylene, most preferably about 20 to 30 percent by weight.
In the preferred embodiment of the synthetic fiber blend, the first fiber component is fibrillated and the second fiber component is present in the form of a twisted bundle of nonfibrillating monofilaments. The twisted bundle is comprised of multiple strands of the nonfibrillating monofilament. Each of the first and second fiber components preferably has a fiber length of about 19 to about 60 mm, and most preferably, has substantially the same fiber length. The first fiber component is preferably about 100 to about 20,000 denier per filament and is preferably present in the synthetic fiber blend in amounts ranging from about 5 to about 50 by total weight percent, and more preferably about 6.7 percent by total weight percent. The second fiber component is preferably about 350 to about 6000 denier per filament, and is preferably present in amounts ranging from about 50 to about 95, and more preferably about 93.3, percent by total weight percent.
The present invention also provides a reinforced cementitious material, such as concrete, comprising the synthetic fiber blend distributed through a matrix of the cementitious material. The synthetic fiber blend may be present in the cementitious material in amounts ranging from about 0.1 to about 2.0 percent by volume, and preferably in amounts ranging from about 0.3 to about 2.0 percent by volume, and most preferably from 0.5 to about 2.0 percent by volume.
The invention also provides a method of forming the synthetic fiber blend. The method includes blending a first fiber component with a second fiber component, wherein the first fiber component is formed of a homopolymer polypropylene fiber and the second fiber component is a nonlibrillating copolymer of a polypropylene and a high density polyethylene. The second fiber component is most preferably twisted to form a non-interconnected bundle of multiple strands of a nonfibrillating monofilament.
A method of reinforcing a material is also provided. The reinforcing method comprises mixing the synthetic fiber blend of the present invention with a cementitious material. A sufficient amount of synthetic fiber may be added to the cementitious material to increase the material""s impact strength. The synthetic fiber is preferably added to the cementitious material in amounts ranging from about 0.1 to about 2.0 percent by total volume, preferably about 0.3 to about 2.0 percent by volume, and more preferably in amounts ranging from about 0.5 to about 2.0 percent by total volume. An addition of synthetic fiber to the material in amounts ranging from about 0.5 to about 2.0 percent by total volume provides an impact strength improvement of at least 6 times that of plain concrete. It is believed that the twisting of the second fiber component into a unique twisted bundle aids in uniform fiber mixing because the twisted bundles separate into untwisted nonfibrillating monofilaments during the mixing process, which in turn is believed to aid in the uniform distribution of the synthetic fiber blend in the cementitious material. The unique twisted bundles permit the fiber blend to be added to the cementitious material at various volume percentages without the clumping and mixing problems experienced with standard monofilament products.